它的工作原理是这样的。想象一下编译器将你的类重写成了这个样子:
class VTable
{
public VTable(Func<Animal, string> eat)
{
this.AnimalEat = eat;
}
public readonly Func<Animal, string> AnimalEat;
}
class Animal
{
private static AnimalVTable = new VTable(Animal.AnimalEat);
private static string AnimalEat(Animal _this)
{
return "undefined";
}
public VTable VTable;
public static Animal CreateAnimal()
{
return new Animal()
{ VTable = AnimalVTable };
}
}
class Human : Animal
{
private static HumanVTable = new VTable(Human.HumanEat);
private static string HumanEat(Animal _this)
{
return "human";
}
public static Human CreateHuman()
{
return new Human()
{ VTable = HumanVTable };
}
}
class Dog : Animal
{
public static string DogEat(Dog _this) { return "dog"; }
public static Dog CreateDog()
{
return new Dog()
{ VTable = AnimalVTable } ;
}
}
现在考虑以下这些调用:
Animal animal;
Dog dog;
animal = new Human();
animal.Eat();
animal = new Animal();
animal.Eat();
dog = new Dog();
dog.Eat();
animal = dog;
animal.Eat();
编译器的推理如下:如果接收器的类型是Animal,则对Eat的调用必须是animal.VTable.AnimalEat。如果接收器的类型是Dog,则调用必须是DogEat。因此,编译器将它们写成以下形式:
Animal animal;
Dog dog;
animal = Human.CreateHuman(); // sets the VTable field to HumanVTable
animal.VTable.AnimalEat(animal); // calls HumanVTable.AnimalEat
animal = Animal.CreateAnimal(); // sets the VTable field to AnimalVTable
animal.VTable.AnimalEat(animal); // calls AnimalVTable.AnimalEat
dog = Dog.CreateDog(); // sets the VTable field to AnimalVTable
Dog.DogEat(dog); // calls DogEat, obviously
animal = dog;
animal.VTable.AnimalEat(animal); // calls AnimalVTable.AnimalEat
这正是它的工作原理。编译器在幕后为您生成虚函数表,基于重载解析规则,在编译时决定是否通过虚函数表进行调用。
内存分配器在创建对象时设置虚函数表。(我的示意图在这方面是错误的,因为虚函数表是在构造函数调用之前设置的,而不是之后。)
虚方法的"this"实际上被秘密地传递作为方法的一个隐式形式参数。
有意义吗?